1----------------------------------------------------------------------
21. INTRODUCTION
3 4Modern filesystems feature checksumming of data and metadata to
5protect against data corruption. However, the detection of the
6corruption is done at read time which could potentially be months
7after the data was written. At that point the original data that the
8application tried to write is most likely lost.
9 10The solution is to ensure that the disk is actually storing what the
11application meant it to. Recent additions to both the SCSI family
12protocols (SBC Data Integrity Field, SCC protection proposal) as well
13as SATA/T13 (External Path Protection) try to remedy this by adding
14support for appending integrity metadata to an I/O. The integrity
15metadata (or protection information in SCSI terminology) includes a
16checksum for each sector as well as an incrementing counter that
17ensures the individual sectors are written in the right order. And
18for some protection schemes also that the I/O is written to the right
19place on disk.
20 21Current storage controllers and devices implement various protective
22measures, for instance checksumming and scrubbing. But these
23technologies are working in their own isolated domains or at best
24between adjacent nodes in the I/O path. The interesting thing about
25DIF and the other integrity extensions is that the protection format
26is well defined and every node in the I/O path can verify the
27integrity of the I/O and reject it if corruption is detected. This
28allows not only corruption prevention but also isolation of the point
29of failure.
30 31----------------------------------------------------------------------
322. THE DATA INTEGRITY EXTENSIONS
33 34As written, the protocol extensions only protect the path between
35controller and storage device. However, many controllers actually
36allow the operating system to interact with the integrity metadata
37(IMD). We have been working with several FC/SAS HBA vendors to enable
38the protection information to be transferred to and from their
39controllers.
40 41The SCSI Data Integrity Field works by appending 8 bytes of protection
42information to each sector. The data + integrity metadata is stored
43in 520 byte sectors on disk. Data + IMD are interleaved when
44transferred between the controller and target. The T13 proposal is
45similar.
46 47Because it is highly inconvenient for operating systems to deal with
48520 (and 4104) byte sectors, we approached several HBA vendors and
49encouraged them to allow separation of the data and integrity metadata
50scatter-gather lists.
51 52The controller will interleave the buffers on write and split them on
53read. This means that Linux can DMA the data buffers to and from
54host memory without changes to the page cache.
55 56Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
57is somewhat heavy to compute in software. Benchmarks found that
58calculating this checksum had a significant impact on system
59performance for a number of workloads. Some controllers allow a
60lighter-weight checksum to be used when interfacing with the operating
61system. Emulex, for instance, supports the TCP/IP checksum instead.
62The IP checksum received from the OS is converted to the 16-bit CRC
63when writing and vice versa. This allows the integrity metadata to be
64generated by Linux or the application at very low cost (comparable to
65software RAID5).
66 67The IP checksum is weaker than the CRC in terms of detecting bit
68errors. However, the strength is really in the separation of the data
69buffers and the integrity metadata. These two distinct buffers must
70match up for an I/O to complete.
71 72The separation of the data and integrity metadata buffers as well as
73the choice in checksums is referred to as the Data Integrity
74Extensions. As these extensions are outside the scope of the protocol
75bodies (T10, T13), Oracle and its partners are trying to standardize
76them within the Storage Networking Industry Association.
77 78----------------------------------------------------------------------
793. KERNEL CHANGES
80 81The data integrity framework in Linux enables protection information
82to be pinned to I/Os and sent to/received from controllers that
83support it.
84 85The advantage to the integrity extensions in SCSI and SATA is that
86they enable us to protect the entire path from application to storage
87device. However, at the same time this is also the biggest
88disadvantage. It means that the protection information must be in a
89format that can be understood by the disk.
90 91Generally Linux/POSIX applications are agnostic to the intricacies of
92the storage devices they are accessing. The virtual filesystem switch
93and the block layer make things like hardware sector size and
94transport protocols completely transparent to the application.
95 96However, this level of detail is required when preparing the
97protection information to send to a disk. Consequently, the very
98concept of an end-to-end protection scheme is a layering violation.
99It is completely unreasonable for an application to be aware whether
100it is accessing a SCSI or SATA disk.
101 102The data integrity support implemented in Linux attempts to hide this
103from the application. As far as the application (and to some extent
104the kernel) is concerned, the integrity metadata is opaque information
105that's attached to the I/O.
106 107The current implementation allows the block layer to automatically
108generate the protection information for any I/O. Eventually the
109intent is to move the integrity metadata calculation to userspace for
110user data. Metadata and other I/O that originates within the kernel
111will still use the automatic generation interface.
112 113Some storage devices allow each hardware sector to be tagged with a
11416-bit value. The owner of this tag space is the owner of the block
115device. I.e. the filesystem in most cases. The filesystem can use
116this extra space to tag sectors as they see fit. Because the tag
117space is limited, the block interface allows tagging bigger chunks by
118way of interleaving. This way, 8*16 bits of information can be
119attached to a typical 4KB filesystem block.
120 121This also means that applications such as fsck and mkfs will need
122access to manipulate the tags from user space. A passthrough
123interface for this is being worked on.
124 125 126----------------------------------------------------------------------
1274. BLOCK LAYER IMPLEMENTATION DETAILS
128 1294.1 BIO
130 131The data integrity patches add a new field to struct bio when
132CONFIG_BLK_DEV_INTEGRITY is enabled. bio->bi_integrity is a pointer
133to a struct bip which contains the bio integrity payload. Essentially
134a bip is a trimmed down struct bio which holds a bio_vec containing
135the integrity metadata and the required housekeeping information (bvec
136pool, vector count, etc.)
137 138A kernel subsystem can enable data integrity protection on a bio by
139calling bio_integrity_alloc(bio). This will allocate and attach the
140bip to the bio.
141 142Individual pages containing integrity metadata can subsequently be
143attached using bio_integrity_add_page().
144 145bio_free() will automatically free the bip.
146 147 1484.2 BLOCK DEVICE
149 150Because the format of the protection data is tied to the physical
151disk, each block device has been extended with a block integrity
152profile (struct blk_integrity). This optional profile is registered
153with the block layer using blk_integrity_register().
154 155The profile contains callback functions for generating and verifying
156the protection data, as well as getting and setting application tags.
157The profile also contains a few constants to aid in completing,
158merging and splitting the integrity metadata.
159 160Layered block devices will need to pick a profile that's appropriate
161for all subdevices. blk_integrity_compare() can help with that. DM
162and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
163will require extra work due to the application tag.
164 165 166----------------------------------------------------------------------
1675.0 BLOCK LAYER INTEGRITY API
168 1695.1 NORMAL FILESYSTEM
170 171 The normal filesystem is unaware that the underlying block device
172 is capable of sending/receiving integrity metadata. The IMD will
173 be automatically generated by the block layer at submit_bio() time
174 in case of a WRITE. A READ request will cause the I/O integrity
175 to be verified upon completion.
176 177 IMD generation and verification can be toggled using the
178 179 /sys/block/<bdev>/integrity/write_generate
180 181 and
182 183 /sys/block/<bdev>/integrity/read_verify
184 185 flags.
186 187 1885.2 INTEGRITY-AWARE FILESYSTEM
189 190 A filesystem that is integrity-aware can prepare I/Os with IMD
191 attached. It can also use the application tag space if this is
192 supported by the block device.
193 194 195 int bdev_integrity_enabled(block_device, int rw);
196 197 bdev_integrity_enabled() will return 1 if the block device
198 supports integrity metadata transfer for the data direction
199 specified in 'rw'.
200 201 bdev_integrity_enabled() honors the write_generate and
202 read_verify flags in sysfs and will respond accordingly.
203 204 205 int bio_integrity_prep(bio);
206 207 To generate IMD for WRITE and to set up buffers for READ, the
208 filesystem must call bio_integrity_prep(bio).
209 210 Prior to calling this function, the bio data direction and start
211 sector must be set, and the bio should have all data pages
212 added. It is up to the caller to ensure that the bio does not
213 change while I/O is in progress.
214 215 bio_integrity_prep() should only be called if
216 bio_integrity_enabled() returned 1.
217 218 219 int bio_integrity_tag_size(bio);
220 221 If the filesystem wants to use the application tag space it will
222 first have to find out how much storage space is available.
223 Because tag space is generally limited (usually 2 bytes per
224 sector regardless of sector size), the integrity framework
225 supports interleaving the information between the sectors in an
226 I/O.
227 228 Filesystems can call bio_integrity_tag_size(bio) to find out how
229 many bytes of storage are available for that particular bio.
230 231 Another option is bdev_get_tag_size(block_device) which will
232 return the number of available bytes per hardware sector.
233 234 235 int bio_integrity_set_tag(bio, void *tag_buf, len);
236 237 After a successful return from bio_integrity_prep(),
238 bio_integrity_set_tag() can be used to attach an opaque tag
239 buffer to a bio. Obviously this only makes sense if the I/O is
240 a WRITE.
241 242 243 int bio_integrity_get_tag(bio, void *tag_buf, len);
244 245 Similarly, at READ I/O completion time the filesystem can
246 retrieve the tag buffer using bio_integrity_get_tag().
247 248 2495.3 PASSING EXISTING INTEGRITY METADATA
250 251 Filesystems that either generate their own integrity metadata or
252 are capable of transferring IMD from user space can use the
253 following calls:
254 255 256 struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
257 258 Allocates the bio integrity payload and hangs it off of the bio.
259 nr_pages indicate how many pages of protection data need to be
260 stored in the integrity bio_vec list (similar to bio_alloc()).
261 262 The integrity payload will be freed at bio_free() time.
263 264 265 int bio_integrity_add_page(bio, page, len, offset);
266 267 Attaches a page containing integrity metadata to an existing
268 bio. The bio must have an existing bip,
269 i.e. bio_integrity_alloc() must have been called. For a WRITE,
270 the integrity metadata in the pages must be in a format
271 understood by the target device with the notable exception that
272 the sector numbers will be remapped as the request traverses the
273 I/O stack. This implies that the pages added using this call
274 will be modified during I/O! The first reference tag in the
275 integrity metadata must have a value of bip->bip_sector.
276 277 Pages can be added using bio_integrity_add_page() as long as
278 there is room in the bip bio_vec array (nr_pages).
279 280 Upon completion of a READ operation, the attached pages will
281 contain the integrity metadata received from the storage device.
282 It is up to the receiver to process them and verify data
283 integrity upon completion.
284 285 2865.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
287 METADATA
288 289 To enable integrity exchange on a block device the gendisk must be
290 registered as capable:
291 292 int blk_integrity_register(gendisk, blk_integrity);
293 294 The blk_integrity struct is a template and should contain the
295 following:
296 297 static struct blk_integrity my_profile = {
298 .name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
299 .generate_fn = my_generate_fn,
300 .verify_fn = my_verify_fn,
301 .get_tag_fn = my_get_tag_fn,
302 .set_tag_fn = my_set_tag_fn,
303 .tuple_size = sizeof(struct my_tuple_size),
304 .tag_size = <tag bytes per hw sector>,
305 };
306 307 'name' is a text string which will be visible in sysfs. This is
308 part of the userland API so chose it carefully and never change
309 it. The format is standards body-type-variant.
310 E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
311 312 'generate_fn' generates appropriate integrity metadata (for WRITE).
313 314 'verify_fn' verifies that the data buffer matches the integrity
315 metadata.
316 317 'tuple_size' must be set to match the size of the integrity
318 metadata per sector. I.e. 8 for DIF and EPP.
319 320 'tag_size' must be set to identify how many bytes of tag space
321 are available per hardware sector. For DIF this is either 2 or
322 0 depending on the value of the Control Mode Page ATO bit.
323 324 See 6.2 for a description of get_tag_fn and set_tag_fn.
325 326----------------------------------------------------------------------
3272007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
328